The present invention relates generally to heat transfer apparatus, and more particularly relates to heat exchangers used in fuel-fired, forced air condensing furnaces.
With the growing need to improve the overall energy efficiency of fuel-fired, forced air heating furnaces, considerable design effort has been directed toward increasing the combustion gas-to-supply air heat transfer capability of their heat exchanger components. Traditionally, fuel-fired forced air heating furnaces have been provided with metal heat exchangers designed to extract only sensible heat from the combustion gases passing therethrough, and transfer the extracted sensible heat to the air being forced externally across the heat exchanger.
Because only sensible heat is withdrawn from the combustion gases, no appreciable amount of condensation of the combustion gases occurs within the heat exchanger during furnace operation. This mode of heat transfer is commonly referred to as a "dry" or "nonrecuperative" process. The combustion gases exiting the heat exchanger, and discharged to atmosphere through a vent stack, are typically quite hot due to the appreciable amount of latent heat remaining therein. Accordingly, a considerable amount of available combustion air heat is simply dumped to ambient, and the overall heat transfer efficiency Of nonrecuperative heat exchangers is generally limited to about 85%.
To capture otherwise wasted latent combustion gas heat, recuperative or "condensing" type heat exchanger structures have been used in which a secondary or "wet" heat exchanger is connected in series with the primary or dry heat exchanger at its discharge side. During furnace operation, the primary heat exchanger performs its usual task of extracting sensible heat from the combustion gases internally traversing it, and the secondary heat exchanger operates to extract primarily latent heat from the combustion gases, thereby considerably lowering the temperature of the gases ultimately discharged into the vent stack. The use of condensing type primary/secondary heat exchanger of this type potentially raises the overall heat exchanger thermal efficiency to about 95%.
However, the corrosively acidic character of combustion gas condensate formed in the secondary heat exchanger (typically drained away via a condensate discharge conduit connected thereto) during furnace operation essentially precludes the construction of the secondary heat exchanger from the same ordinary metal used to form the dry process primary heat exchanger. Accordingly, various proposals have been made to form the secondary heat exchanger from stainless steel. It has been found, however, that even stainless steel is not entirely suitable, particularly in operating environments where the combustion gas condensate form hydrochloric acid. This particular corrosive attack problem may be to a large part overcome by using, for example, a stainless steel molybdenum alloy, but the high cost of this material makes it unsuitable for furnace heat exchanger applications.
Another material considered for use in the construction of the secondary heat exchanger portion of a recuperative heat exchanger of this type is plastic, and various types of plastic have been investigated and evaluated. For a variety of reasons, though, plastic secondary heat exchangers have not proven to be entirely satisfactory despite their typically high resistance to acidic corrosion.
For example, the construction of plastic heat exchangers has heretofore required complex manufacturing techniques entailing high tooling and assembly costs, a variety of secondary operations, and a relatively lengthy assembly time per unit. Additionally, compared to their metal counterparts, conventional plastic secondary heat exchangers have often been susceptible to thermal shock and have exhibited other undesirable structural weaknesses.
Moreover, due to the considerably lower heat transfer capability of plastic compared to metal, a plastic heat exchanger of conventional construction tends to be considerably larger than a corresponding metal heat exchanger of the same heat transfer capacity. Particularly in a furnace of an otherwise compact configuration, this is a decidedly undesirable characteristic. Attempts to construct plastic heat exchangers in a manner permitting them to occupy generally the same overall volume as their metal counterparts has typically resulted in an unacceptably high air pressure drop across the plastic heat exchanger.
From the foregoing, it can readily be seen that it would be highly desirable to provide an improved plastic secondary heat exchanger, for recuperative use with a metal primary heat exchanger in a condensing furnace, that eliminates or at least substantially reduces the above-mentioned problems, limitations and disadvantages heretofore associated with plastic heat exchangers in this general application. It is accordingly an object of the present invention to provide such an improved plastic heat exchanger.